text: Another category of multipass cells is generally referred to as circular multipass reflection cells. They were first introduced by Thoma and co-workers in 1994.  Such cells rely on a circular arrangement of mirrors. The beam enters the cell under an angle and propagates on a star-shaped pattern (see picture on the right). The path length in circular multipass cells can be varied by adjusting the incidence angle of the beam. An advantage lies in their robustness towards mechanical stress such as vibrations or temperature changes. Furthermore, circular multipass cells stand out because of the small detection volumes they provide. A stable beam propagation is achieved by shaping individual reflection points to form a non-concentric mirror-arrangement.
  In a special case, a circular mirror is used, allowing continuous adjustment of the angle of incidence. A drawback of this circular cell configuration is the inherent concentric mirror arrangement which leads to imperfect imaging after a large number of reflections.